The present invention relates to a multiplex type transmitting apparatus which is arranged to hold various kinds of transmission synchronous systems of an SDH (Synchronous Digital Hierarchy) and a SONET (Synchronous Optical Network).
The conventional synchronous digital transmitting systems include a SONET synchronous transmitting system defined on the ANSI T1.105 and an SDH synchronous transmitting system defined on the ITU-TG.707 (1995).
For both of these transmitting systems, a concept of a layer such as a path, a line (M-section), or a section (R-section) and a way of use of an overhead are commonly set to these two transmitting systems.
FIG. 1 is a table showing a comparison of a way of use of each overhead of two kinds of synchronous digital transmitting systems. In the promotion of the overhead in each of the synchronous digital transmitting systems, the differences between the SONET transmitting system and the SDH one is only the following four points. First, the former system uses a C1 byte of a section overhead for specifying an OC-1 multiplex number of an OC-M signal, while the latter system uses the C1 byte of the section overhead for specifying an STM-1 multiplex number of an STM-M signal. The second different point is the way of use of the JO byte of the section overhead. The third different point is the way of use of the S1 byte of the line overhead. The fourth different point is the way of use of SS bits which correspond to fifth and sixth bits of an H1 byte of a pointer.
The SDH transmitting system gives the C1 byte a multiplex number expressed at an STM-1 basis, while the SONET transmitting system gives the C1 byte a multiplex number expressed at an OC-1 basis. It means that both of the transmitting systems have respective transmission values. However, the SONET transmitting system is regulated to ignore the received C1 byte, while the SDH transmitting system has no regulation about the received C1 byte.
The JO byte is used as a section trace (R-section trace). The SDH transmitting system has a regulation about the way of use of the JO byte defined in the ITU-T G.707, while the SONET transmitting system has no regulation thereabout.
The low-order four bits of the S1 byte is used for a Synchronization Status Message indicating a network synchronous state. The code values of the four bits are respective in both of the transmitting systems.
In turn, the description will be oriented to the SS bits of a pointer. The pointers positioned at the H1 and the H2 bytes of the overhead are used each for indicating a head phase of a path. As shown in FIG. 2 which indicates the structure of a pointer portion of the overhead, both of the transmitting systems have respective ways of use for the SS bits of the H1 byte (fifth and sixth bits of the H1 byte). Concretely, the SDH system is defined so that the SS bits indicate an AU type and are defined as "10", while the SONET system is undefined. In the SONET system, the undefined bit is "0", so that the SS bits are made to be "00".
The SONET transmitting system is arranged to ignore the SS bits in interpreting the pointer value, while the SDH transmitting system is arranged to contain the SS bits in the condition of interpreting the pointer value as described in the ITU-T G.783 ANNEX C(1996).
Herein, consider that a certain multiplex type transmitting apparatus is connected to the SDH system transmission path and the SONET system transmission path. For example, the fast transmission path employs the SONET system, while the slow transmission path employs the SDH system. At this time, the disadvantageous factor impacted on a main signal is only the difference of the SS bits among the foregoing four differences. Later, this factor will be discussed concretely. The slow transmission path is a signal transmission path in low bit rate of data before the multiplexing, and while the fast transmission path is a signal transmission path in high bit rate of data after the multiplexing. Hereinafter, the signal in low bit rate and the signal in high bit rate are called "slow signal" and "fast signal", respectively.
Assume that the SONET system multiplexing apparatus provided with a pointer processing section receives the SDH system SS bits ("10"). In this case, the SS bits are ignored, so that the SS bits give no impact on the main signal. On the other hand, assume that the SDH system multiplexing apparatus provided with the pointer processing section containing the SS bits as the interpreting condition of the pointer value receives the SONET system SS bits ("00"). In this case, even if the received data is normal, the pointer processing section operates to detect a lost of pointer (AU-LOP) resulting from a mismatch of the SS bits. If an AU (Administrative Unit) detects an AU-LOP, an alarm signal (AU-AIS) is inserted into the AU. As a result, the received data disappears. Herein, the insertion of the AU-AIS means that the AU pointer and all the bits of a higher-order virtual container are made "1's".
On the other hand, the differences of the JO (C1) byte and the S1 byte do not have any impact on the main signal, such as disappearance of the received data.
As set forth above, the SONET system has a different way of use of the JO (C1) byte, the S1 byte, and the SS bits (fifth and sixth bits of the H1 byte) of the H1 byte of the pointer from the SDH system. If, therefore, any one of the SONET and the SDH transmitting systems is connected as a counterpart of the slow signal transmission path or the fast signal transmission path, each time the counterpart is connected, it has been necessary to at least select the SS bits of the SONET system or those of the SDH system to be transmitted by setting the SONET system signal or the SDH system signal from an operation system or a local craft interface. Or, in some cases, it has been necessary to select the JO (C1) byte and the S1 byte of the SONET system or those bytes of the SDH system in addition to the above-mentioned selection of the SS bits together.
In a case that the multiplex type transmitting apparatuses connected over a border of an international network have difficulty in determining that the counterpart of the slow signal transmission path or the fast signal transmission path is the SONET transmission system or the SDH transmission system, the transmission of the proper SS bits is disallowed to be assured. In some cases, the counterpart of the SDH system may induce the disappearance of data resulting from the mismatch of the received SS bits as mentioned above.